Hmm, there's a number of possible reasons for this.

What is the make/model of laptop, how old is it, specifications - graphics chip, cpu, memory, OS, etc.

I would begin by checking the following:

Has there been any updates to the OS/drivers recently?
Any new applications installed?
Check for Background processes.
Check temperatures of Graphics Card and CPU. (Clear out any air vents, etc)
Run a game file repair.
Run diagnostics on memory and disks.