Hard to tell without seeing the data. Share a link to the activity, and we may perhaps try to help.
Did you get the GPS lock sign before starting the run?
Possible reasons are an expired EPO/CPE file (ephemeris data), insufficient GPS signal (high-rise buildings, walls, terrain, vegetation, ...), hardware failure, a firmware update gone wrong, ...
I recommend syncing and then soft-resetting the watch (holding the light button till the watch shuts down - no data will be lost), connecting the watch over the USB cable to a computer with Garmin Express, and checking for updates. Then make another run, and do not start before getting the GPS lock.
