Diagnosis
You've got a weak signal. The battery in the remote is probably dying, or something is causing radio interference between the remote and the opener's antenna.
How to Fix It
Replace the battery in your handheld remote. It uses a standard CR2032 coin cell. Pop the old one out, wait 10 seconds, and put the new one in with the positive (+) side facing up. Then, you need to re-sync it. Press the 'Learn' button on the opener motor unit once, then within 30 seconds, press and hold the button on your remote until the opener light blinks or you hear a click.
If That Didn't Work
Check the antenna on the motor unit. It's a thin, hanging wire. Make sure it's fully extended and not coiled up or tucked behind metal. It should be dangling straight down. Also, look for new sources of interference: a new LED light fixture in the garage, a cordless phone base station, or even a metal shelf that's now between the remote's path and the opener.
Heads Up
If you have multiple remotes, test them all. If only one has short range, it's that remote's battery or a fault in that remote. If ALL remotes and the keypad have short range, the problem is with the opener's antenna or its receiver circuit.