Where is power supply unit located & how do you check it

Ignore any and all information from "Anonymous232" as it is absolutely INCORRECT.

The power supply unit ("PSU") will be located on the rear of the chassis at the top. You will find one of two types of PSUs located there - depending on the original configuration.

1.) A standard ATX power supply. This consists of a single square box with a single receptacle plug that accepts a C13 connector.

2.) Two redundant, modular PSU units that can be removed or inserted without interrupting power to the other unit. These will each also have a receptacle that accepts a C13 connector.

You will want to ensure that the power cables are firmly seated in the plug on both ends. (If the server has redundant PSUs, check both cables.)

POWER CONNECTORS
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The connectors on either end will almost always be one of the following:
(NOTE: Attaching a link for visual aid. Use it: Power Connector Guide)

NEMA 5-15-P: Your standard, three-pronged plug found on most appliances. This would plug into any standard 15A/20A outlet on either a wall or a power distribution unit. (A PDU - such as an Avocent PM20i - is normally found in a server cabinet in a small office or data center.)

The other end will, in most cases, be a C13 connector. (The typical plug type that plugs into the rear of nearly every type of computer and most network devices.)

C13: This is the rectangular connector that has two angled edges on both ends of the same elongated side.

C14: This is a bit of a 'reverse' connector which accepts the C13 plug type. This is typically used in server cabinets to connect servers to a PDU.
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ASSESSMENT
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The power cables that you're using are likely NEMA 5-15-P to C13. This is the usual, everyday cable for computers.

1.) Check your wall outlet. Is it a breakered GFCI wall outlet? Make sure the tiny breaker is not tripped. If so, press the button back in to reset the breaker.

2.) Are you using a surge protector? If this surge protector was working then, suddenly, your server won't turn back on... I advise you to replace the surge protector as it was significant surge that would have created this situation for you. (Even if other devices plugged into it are still working.) This surge protector is now compromised.

3.) If you are not using a surge protector and it is not a GFCI outlet with a tripped breaker button then you should go to your electrical box and make sure that the breaker for that wall socket is not tripped. (Again... If other devices are working on that outlet then that will not be your issue.
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DIAGNOSIS
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If you have ensured that none of these are the problem then you likely have one (or more) of the following issues:

1.) Both PSUs are failed. Check for any orange (failure) LEDs or no lit LEDs at all while having power connected. If no green LEDs or all LEDs on the PSUs are dark then they are likely failed.

2.) Having BOTH PSU fail is an unlikely scenario. This means that your motherboard is more likely to be the point of failure here. You can only tell this by swapping the PSUs.

3.) If using redundant PSUs, they are being inserted into a type of backplane housing. This backplane, depending on it's design, is also a potential point of failure. (Although a much less likely one. Very rare.) You can replace this if you have a spare.
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TIPS
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If you're going to start swapping parts, it'll likely cost you more than this machine is even worth anymore. This server can typically be had for $50 or less on ebay these days. If you're going to buy parts for it, you might as well buy an entire, working chassis and strip the parts from that to repair yours.

This issue is old, though. I hope I haven't wasted my time... LOL

Is there a power-cord connecting from an electrical outlet to some socket on the computer? If so, then the "female" end of the power-cord connects to the external "socket" of the power-supply.
Open the computer, and trace the "inside" of that "socket", where it connects to the power-supply.