For most players (specially non professional) this happens when they don't play cricket for some times. That's why professional players keep practising on off-days as well. The main reason is lack of confidence in throwing and losing the grip on the ball. Either they think they will bowl full toss or very short deliveries. That's why they bowl either very short or full toss respectively. To get back your confidence you may try this:
- Start practising with throwing the ball to a good length area on the wicket without any run-up or moving your hand round (i.e. just normal throws). Don't care of the speed.
- If you think you are able to throw in a good length area, try to grip the ball same as you do while bowling and keep throwing. Improve the speed upto your actual speed.
- Now try to bowl the ball without run-up and maintain the speed. This is very important. If you are able to bowl in a good length area, you will able to bowl in the same area while bowing with run-up.
- Now try to bowl with run-up. If you are not able to bowl in a good length, you may start with small run-up then improve it to your actual run-up.
- You will probably be able to bowl in a good length with proper grip on the ball now.
Note: I am not a professional trainer. I too am not able to play regular cricket. And this is what I do when I lose my line, length and grip and mostly this works.