Emachines T2682 won’t power up?

The emahines T2682 had been working fine for several years. Normally the machine is left turned on. I power down very infrequently.

Went to use the PC tonight and it was powered down. Pressing ON/OFF switch does not power up the macine.

Would this be a power supply problem or a mother board problem? There are many similiar posts to this problem but almost all talk about getting some indication the PC is getting some power. My machine is getting none, no fans running, no lights on, nothing.

The machine currently has a BESTEC ATX-250-12E PS. How do I test the PS to see if this is the problem.

Thanks for the help.