If you hit a below ground line
Even if it looks like a little nick in the utility line, it's very dangerous and needs to be fixed immediately. Damaged phone lines or fiber optic cables can disrupt 911 emergency service. If you do hit a line, do not bury it. Here's what to do:

Electrical

Call 911 and your electricity utility immediately.
Keep your co-workers back.
Do NOT touch the equipment and the ground at the same time.
If you're on the equipment, do NOT get off (unless it's on fire.)
If you must get off, jump as far as you can and land with your feet together. Shuffle away with feet together and on the ground.

Gas pipeline

Call 911 and your gas utility immediately.
Call your supervisor.
Keep people away.
Keep all ignition sources (like vehicles) away, as this can cause gas to ignite.

Utility

Notify your supervisor and the utility owner immediately.
If you cut a fiber optic cable, don't look into the end of it. This can cause serious eye damage even if the light source is not visible.