why is sometimes i get this message error during browsing my site ? but when i refresh it comes back , what causes this error ?

Service Temporarily Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
Apache/1.3.33 Server at www.example.com Port 80

the server is reaching capacity or is paused for maintainence,

all the sites hosted on the server may have poorly designed scripts
it may just be maxxed out
or they may be maintaining it

Just like the error message says, its plain (almost) language,
contact your host and find out which, and when it will be finished

so there is no solution for it ?
is there any php code to automatically refresh the url if we get this error ?

it might also indicate a problem with your account. Please log in your to your Web Hosting Account and check if there are any active tickets or server notices for your account in your Help Desk.

is it 503? the error code?

there's no active tickets or server notices in my account

yes it's 503 error code

is there any code can be set into .htaccess to make the requested page to refresh if this error message appear ?