Well if u can boot bios verify that u still have a working hardrive by checking if bios still detects it, after that u can try to use a windows cd to do a repair on it, maybe some sistem files are corrupted, if this fails try using a linux live cd to run some hardware diagnostics ( if u have 2 many bad sectors it might not load windows properly ).
If hard drive is ok and repair doesnt work u will have to do a clean instal .