A blinking green light on the motherboard can indicate either a power supply problem, or a memory problem. Depends on the model of the motherboard. It can also indicate that the ATX cable isn't properly seated on the motherboard (which would by default be a power supply problem, but is specific enough that I wanted to mention that).

If you've recently opened the case and fiddled around with the components, reopen the case and make sure all connectors are firmly seated and that any locking hasps are properly locked.

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