new gpu pc not booting

Got new psu finally. Seasonic focus gold 550fm. Everything is working perfectly.

I does not really have to be 500W+, if it's a good quality PSU. The GPU manufacturer's recommendations leave significant amounts of headroom for power hungry components. Your CPU's TDP is 95W, the RX 550 has a TDP of 50W, so your total power draw from the wall is likely below 200 W. Anything from 400W upwards should be more than enough as long as it's a quality unit.

I know the power requirements of that RX 550 aren't big nor are they a big jump from that GT 520, but your PSU is apparently one of the lowest quality brands out there. Tom's Hardware's PSU tier list puts the Diablotek brand in Tier 5, which is described as follows:

"Avoid IMMEDIATELY. These units are highly unsafe to use. No such protections added, very thin gauge wiring used, false advertising and too much to list. Reference to a higher tiered unit for a better, money saving and a much safer unit. For your safety's sake, please don't order or pick one up for use in your system. These units are a potential fire hazard and could even kill you, let alone your system."

My guess is that the 500W rating of that PSU is "optimistic" and that the RX 550 might have been the final straw that pushed the power consumption of your system beyond the capabilities of that unit. Do you have access to another PSU that you could use for testing?

just got a msi rx 550 4g oc edition card. When installed pc will not boot up. All fans come on keyboard lights up, then it just sits and keeps resetting. Screen never comes on. Have tried multiple eufi settings, check mb drivers. plz help.
pc specs
MB: fm2a78m-itx+
cpu: amd 880k
psu: diablotek el series 500w
ssd: crucial 275g
ram: 8g (don't recall brand at moment)
current gpu: nvidia gt 520