Try running your machine in from your boot disc in Live mode.
If that starts ok then it is worth looking at RAM and hard drive health, there are tools on the Live session that will let you run a checkdisk and get smart disk reports or you can run something like Dr.Parted or Rescatux.
I am afraid that this information is non-specific as you have not provided information that could be vital to a successful repair.
Was your failure directly after updating and restarting or did your PC just randomly fail to boot?
Were there any noticeable faults before it failed?
Had you been doing anything that might have contributed to the boot fail?