I was adding a ram module to my computer. When I started ii I smelt burning. I replaced the power supply and now the light goes on but nothing happens. Will I have to buy a new motherboard?

It depends if you've broke your Motherboard or not. More than likely you have tried to install DDR RAM into a computer which doesn't support DDR. There are loads of different types of RAM out there, you're computer will probably only support one to five types. Have a look at your old RAM sticks, see if you can see anything such as "DDR2" or "SDRAM" or anything else. Then write back to tell me your findings.

The thing is, the ram I was putting in was previously in the machine so I don't know what could have happened

Make sure the RAM you were putting in isn't used (such as did you buy new RAM to replace thos eones because they were used up), and make sure you put the RAM in the correct way round. Other than that I don't know much.