Is there a limit to how long a wallet can be offline?
Any unspent bitcoins against addresses in your old wallet should be retrievable.

The client won't show transactions that haven't been downloaded to your copy of the blockchain yet. There should be an icon in the lower right corner of the Bitcoin-QT client that indicates progress on downloading the blockchain. It can take a few hours (there's around 2.5GB to download, and it downloads them slower than you'd expect because the client verifies the blocks as it goes).