Never looked at it again and now my balance shows zero bitcoins. Can anyone help me to get my two btw back? Should I just make a backup?
As Lucius mentioned, mycelium server can often not show the up-to-date balance.
Do you see a transaction history? If not, chances are very high that his is a synchronization issue.
You can then either try to import your seed into a different wallet or simply wait a few hours / a day.
However, if you see a transaction history, look for the most recent transaction. If it is an outgoing one, your wallet might be empty.
If it is a receiving one, check the address on a block explorer to see whether (and how much) coins it still holds.
Before resetting your wallet, make sure you have a proper backup of your seed words!