The other day i turned on my pc and all the normal sounds of fans cranking etc came on but the screen stayed blank... i reset a few times before realising somethign was seriously wrong and taking off teh side cover. When i did this is saw that my northbridge heat sink had fallen off ( the pins attaching it had broken) now ive trauled some other forums and have found this is a common problem and most ppl recommend reattaching the sink by either soldering the pins back in or using acrtic silver thermal adhesive glue to reattach these options are cheap =D but be4 i follow them up i need to know if my northbridge is fried or not...any ideas on how to check...i mean its no good reattaching heat sinc if it is... if my north bridge is fried can it be replaced? or will i need a completely new mobo?

If it works for a few minutes before dying, then it probably works fine and the heat is just interfering with the tasks. Restoring the heat sink should return it to normal.

If it doesn't work at all, then something melted and it will never work again. You can reattach the heatsink and see if that works (since it's cheap to do anyways), but you're probably going to be disappointed. You will have to replace the entire motherboard if that's the case.