|Make/model would be useful for pholks to assist more easily?|
If a laptop - check to see if all cooling vents are clear; and also that the cooling fan is actually doing its job?
If a desktop/tower - check similarly and also ensure the cooling fan (there may be even be two) is actually running. One fan may be within the psu and another (if present) separate within the case back plate assembly etc. Usually there is just one - and it's usually within the psu housing itself.
Any messages when it boots and/or reboots that aren't (or weren't until now) present normally?
Have you made any changes to the system at all - components etc?
And again make/model is most useful here.