1. Check your BIOS, make sure your system still sees the hard drive. Also, make sure your hard drive is the first drive in the boot sequence.
2. If all this is okay, your MBR (master boot record) got corrupted/wiped.
The problem is that I haven't got the CD. When I bought my PC, Windows was included on it. It happened after I've changed power socket, and maybe that guy who fixed it has ruined something. The funny thing is that it worked in about an hour, but then crashed and got blue screen. Now it will not start.
when you say power socket, are you referring to the power input port on the laptop itself? or are you referring to the power pack itself? sometimes changing that port can be a hassle which requires the tech to strip the laptop down to basically nothing.
i'd take it back to him and make him fix it. either the drive isn't plugged in properly, it's dying, or someone/thing changed the boot order or the boot sector.
That message means that your BIOS can't find a bootable disk, which means something is up with your hard drive. If you can boot some kind of boot disk from either cd or usb, you should be able to take a peek and see if your hard drive is there and if it has anything on it.
Myself, I'd just grab a Linux live disk from Ubuntu or Debian, burn a cd or copy to a thumbdrive then boot, and then sort out my disk problems. That's probably a bit beyond you but think about what has changed... did the disk work yesterday but not today? It might be a failing hd. Did you update a bunch of software and then the reboot didn't work? Something trashed your mbr and/or partition table. Did the guy who repaired it give it back and then it doesn't work? He might have trashed your hd or given you a new one.
Well, unless you're at least a bit technically inclined, take it to a pro.
If you got a BSOD and then you laptop stopped booting, it's probably a sudden failure of the hard drive. Assuming the drive is not completely dead, you might be able to get some data from it using a LiveCD of Ubuntu or some other distro and copy the data to a flash disk.