The Panasonic Lumix DMC-FZ5 is an SLR
type camera - ie it looks and handles a bit like an SLR, but the Lens
is fixed (so don't try to unscrew it in a vice!
)
The small amount of dust that may get into the lens when a filter is removed shouldn't stop it focusing (unless you get some
major grit / sand in there) so try the following:
1) First make sure you haven't switched it to manual focus by mistake - there should be a setting for this in the menus somewhere.
2) Check your batteries - there may be enough left to power the screen etc, but possibly not the motor for autofocus.
3) Even if the batteries seem fine, switch the camera off, remove then replace them and switch back on again - if it still doesn't work, try doing a function reset (or something similarly named, to reset to all original settings).
4) Check if its still under warranty, and take it back to the shop you bought it from if it is...
5) Start looking for your nearest camera repair shop...