the speedometer is operated mechanically by a cable that comes up from the transmission, through the firewall, and plugs into the back of the speedometer. It's clipped in place to the back of the speedo, and it's fairly easy for it to come unclipped. if you get under the dash (head and shoulders on the floor, butt and legs sticking out the door) you can usually feel up behind the dash to see if the cable is connected. If it's come loose, just push it back in and see if you can feel that the clip has engaged, 'cause otherwise it will come out again after a few miles.

if the cable is connected at both ends and the speedo doesn't work, the answer almost certainly is ... a bad cable.

probably coincidence that the problem occurred when it did, though I suppose you might have accidentally pulled on the cable while changing the fuel filter.