Based on the very few details you provide it could be everything. Is that a new problem? Are you using the latest software version? What kind of activity was it and did you have good GPS coverage? You can't fix a problem before analyzing the root cause.
I don't know much about the elevation calibration situation, but since I had read some of the posts a few months back about it I have made a point to sync watch when I am about to head out the door (or better yet, calibrate while outside just before run if I have my phone on run/ride). Temperature effects the sensor greatly, sadly it isn't calibrated much by the Temp sensor. You can see fake hills/valleys in winter when watch is under jacket or exposed (temperature goes hi/low also) also if you pause for a while in outhouse then restart... usually shows a hill.
Before you start an activity (especially in new area) make sure you have a good GPS lock (ideally allowing watch sensor to pickup temperature) and then also sync with the Garmin App before starting run. The user manual on Garmin has some info on calibration of sensor... but if I remember, not much in the way of details or what to do.
