The bass clef changed to treble clef in the middle of the music sheet. Do both hands play above the middle c?
Yes, you play with both hands above the middle C. This is quite common for instruments that are written in both clefs, not only in piano.
This will help you, because if the bass clef goes really high, it is really hard to read all those extra lines. So, since you are basically in the treble clef, the music usually shifts clefs.
It is pretty common to change clefs in the middle of a piece. There are 4 fairly common clefs (treble, alto, tenor, bass), and which one a piece is written in really depends on where the notes are on the clef. Each clef puts middle C in a different place on the staff.
If the left hand is centered above middle C, it makes more sense to use a treble clef than a bass clef. Read it just like you would a normal treble clef. If it's a short passage (just a few notes) you may be crossing hands; in that case, the left hand part would be written higher than the right hand part. Otherwise, you'll probably find the right hand high on the treble clef or even 8va (octave higher).
You may also find the right hand in the bass clef, though that is not as common. The left hand moving into treble range is pretty common. The other clef doesn't change at all.