Check the error logs (PHP, Apache). If that doesn't help, use Fiddler or FireBug for example to check whether you perhaps have created an infinite loop.

FyI when you check your Apache logs, the most recent data will be at the bottom not the top.

[Fri Sep 03 07:59:16 2010] [notice] Apache/2.2.11 (Win32) PHP/5.3.0 configured -- resuming normal operations
[Fri Sep 03 07:59:16 2010] [notice] Server built: Dec 10 2008 00:10:06
[Fri Sep 03 07:59:16 2010] [notice] Parent: Created child process 3268
[Fri Sep 03 07:59:19 2010] [notice] Child 3268: Child process is running
[Fri Sep 03 07:59:19 2010] [notice] Child 3268: Acquired the start mutex.
[Fri Sep 03 07:59:19 2010] [notice] Child 3268: Starting 64 worker threads.
[Fri Sep 03 07:59:19 2010] [notice] Child 3268: Starting thread to listen on port 80.
[Fri Sep 03 08:05:13 2010] [error] [client 127.0.0.1] File does not exist: C:/wamp/www/favicon.ico
[Fri Sep 03 08:05:15 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Product_Name in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 17, referer: http://localhost/web_test/
[Fri Sep 03 08:05:15 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Image in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 19, referer: http://localhost/web_test/
[Fri Sep 03 08:05:15 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Branch_Name in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 21, referer: http://localhost/web_test/
[Fri Sep 03 08:05:15 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Quantity in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 23, referer: http://localhost/web_test/
[Fri Sep 03 08:05:15 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Price in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 25, referer: http://localhost/web_test/
[Fri Sep 03 08:05:16 2010] [error] [client 127.0.0.1] File does not exist: C:/wamp/www/favicon.ico
[Fri Sep 03 08:05:41 2010] [notice] Parent: child process exited with status 255 -- Restarting.
[Fri Sep 03 08:05:48 2010] [notice] Apache/2.2.11 (Win32) PHP/5.3.0 configured -- resuming normal operations
[Fri Sep 03 08:05:48 2010] [notice] Server built: Dec 10 2008 00:10:06
[Fri Sep 03 08:05:48 2010] [notice] Parent: Created child process 5504
[Fri Sep 03 08:05:49 2010] [notice] Child 5504: Child process is running
[Fri Sep 03 08:05:49 2010] [notice] Child 5504: Acquired the start mutex.
[Fri Sep 03 08:05:49 2010] [notice] Child 5504: Starting 64 worker threads.
[Fri Sep 03 08:05:49 2010] [notice] Child 5504: Starting thread to listen on port 80.
[Fri Sep 03 08:05:49 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Product_Name in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 17, referer: http://localhost/web_test/
[Fri Sep 03 08:05:49 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Image in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 19, referer: http://localhost/web_test/
[Fri Sep 03 08:05:49 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Branch_Name in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 21, referer: http://localhost/web_test/
[Fri Sep 03 08:05:49 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Quantity in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 23, referer: http://localhost/web_test/
[Fri Sep 03 08:05:49 2010] [error] [client 127.0.0.1] PHP Notice: Undefined index: Price in C:\\wamp\\www\\web_test\\stock control system\\index.php on line 25, referer: http://localhost/web_test/
[Fri Sep 03 08:06:00 2010] [notice] Parent: child process exited with status 255 -- Restarting.
[Fri Sep 03 08:06:00 2010] [notice] Apache/2.2.11 (Win32) PHP/5.3.0 configured -- resuming normal operations
[Fri Sep 03 08:06:00 2010] [notice] Server built: Dec 10 2008 00:10:06
[Fri Sep 03 08:06:00 2010] [notice] Parent: Created child process 5880
[Fri Sep 03 08:06:01 2010] [notice] Child 5880: Child process is running
[Fri Sep 03 08:06:01 2010] [notice] Child 5880: Acquired the start mutex.
[Fri Sep 03 08:06:01 2010] [notice] Child 5880: Starting 64 worker threads.







this is the log eror can someone plz tell me what i must do ?

There is no indication here as to why the server would stop.

then what might be the problem ?

Hard to say without being able to test your code.