computer problem

so, my computer died or so.
it doesn't work.
*dies*
i wanted to install another harddisk, but after connecting it he didn't want to start again. nothing worked. so i took it off again and just used my old one. but it wouldn't work as well and it made strange sound. a annoying beeeeeeeping from the motherboard (we suspect). it can't be the harddisk, it can't be the cables. so, it must be the motherboard, but i have now clue what is wrong or how it got fucked up.
i hope nothing is seriously broken *sigh*
but anyway it will cost money to bring it to the computerstore. *sighs some more*
any ideas?