Update your post to include full system hardware specs and OS information.
Include PSU: make, model, wattage, age, condition?
BACKUP: ensure that all important data is backed up at least 2 x and stored elsewhere in different locations off of the computer. Be sure that the data is recoverable and readable.
Look in Reliability History and Event Viewer for any error codes that correspond with the screen going black and/or the PC reboots.
One immediate thought is a loose connection.
Power down, unplug, open the case.
Clean out dust and debris.
Verify by sight and feel that all cards, connectors, RAM, and jumpers are fully and firmly in place.
Post accordingly.