I have a Bernina 135S and I managed to get a pin caught in between the plate and the feed dogs. I removed the plate, took out the pin and put it back together although now my feed dogs only move up and down rather than pulling the fabric through the machine. I have taken the back off the machine but can't seem to locate the issue, can anyone help me?

Check whether the feed dogs are coming above the stitch-plate at their maximum height. They should be about 1mm above the stitch plate when driving the material under the foot. I assume, by the way, that you haven't got the feed-dog drop button pushed-in at the handwheel end !!! If the feed-dogs are lower than 1mm, or not above the stitch-plate, the dog cam cam-foller will need adjustment. You have to remove the front cover to get at this and it should be done with a special gauge, so really its a service agent job, however, for what its worth;
With the front cover removed to gain access to the free-arm mechanism, look to the right of the hook race and on the end of the bottom drive shaft there are two black plastic cams . The right-hand one regulates the movement of the dogs front-to-back in association with the stitch-length stepper-motor, which drives the upper shaft in the free-arm. The left-hand cam regulates the up-down movement of the dogs and the cam follower (a piece of right-angled plated metal) has to be adjusted to determine the maximum height of the dogs aboe the stitch plate. The cam follower rubs against the top edge of the dog cam. There should be a small spring that pulls the cam follower down onto the cam at the front - if that's missing, the feed-dogs probably wont move quite correctly. To adjust the dog height, turn the hand-wheel until the feed-dogs are at their maximum height, unscrew the small screw in the left-hand side of the cam follower a couple of turns (dont, for goodness-sake, take it out !) - the feed-dogs should then pop-up above the stitch-plate. You now have a problem without the proper gauge ! You need to gently push the dogs back down against spring pressure until they are the correct height above the stitch-plate and tighten the screw on the cam follower. This should, at least, get over the problem - it wont be exactly right without the gauge and there is another adjustment that should be done to ensure the front and back of the dogs are at slightly different heights, but it should be good enough without this.

Good Luck !!!