Fan speed resistor pack has failed. Usually the non-replaceable thermal fuse
blows. If you are used to soldering electrical components you may be able
to replace just the fuse, if not then the complete pack must be changed.

Google this news group for information on how to fix it.

HTH

Chris

Normal cause 1 - blown thermal fuse in the motor control module. The
bottom 3 speeds are controlled by a bank of resistors with an in-line
thermal fuse. The top speed just places the motor directly across the
12V.  The thermal fuse is built into the motor control module.

It can be found in the behind the glove box . Flex the plastic sides of
the glove box inwards with a tiny bit of force  and you can swing it out
of the way). The module will be found somewhere by the blower fan motor.
Held in place with one screw and it has a removable electrical
connection.

A couple of posters to this newsgroup have said that they purchased a
new module from the dealer at around 15 $ US - or the equivalent in
Euros - and they replaced it themselves.

For a picture of the module plus circuit diagram see:
<http://www.amac.f2s.com/Focus1/>

The module may be black rather than blue.

If you have some electrical/electronic knowledge and tools then it will
be possible to cut out the thermal fuse - leaving most of the connecting
leads behind - and soldering in a new fuse. This DIY repair may save
you, say, $13 , or so but maybe not worth the hassle if the module is a
reasonable price from the dealer.

It is not unknown for the selector switch to fail - but this is less
likely with the symptoms you describe.