Make sure the altenator is charging. If it's not charging the tach won't recieve a pulse count.
Check your fuses, and the connections at the altenator for the smaller wires. Some times it's as simple as a spade not making good connection on the altenator. Simply wiggling the wires at the altenator could fix it temporarily. If thats the case it will show you where to look and check.
If you rule out the altenator side or fuse issue, then it's probably on the tach side. Could be an issue with the power, ground, or sending wire going to it.
If it's still under warranty, having the dealer take care of it, is an option as well