electrical GFI replaced old plug
See how the power comes in via Cables?  Each cable has 1 black wire, 1 white wire and probably a ground wire? 

One of those cables is supply.  The other one is onward power to your light or whatever. 

I see no reason a light needs to be under GFCI protection.  The LOAD terminals are not for using, unless you know exactly what you are doing and intend to GFCI-protect the onward loads.  Protecting hardwired lights is silly, that's ground's job. 

Place both blacks on GFCI "LINE" brass screw.

Place both whites on GFCI "LINE" silver screw. 

Most GFCIs allow 2 wires in back-wires directly under the screw; if not, then pigtail the 2 black wires and 2 white wires.  Crank the screws down quite hard. 

Now, check other receptacle sockets that lost power during this situation.  If they are in a place where GFCI is required, do a GFCI test on those. If it trips a GFCI (maybe this one), mark the socket "GFCI Protected". They make stickers for that purpose. Otherwise make sure to fit another GFCI there, or revisit the "know what you're doing/intend to protect onward loads" thing.