Your question is a little bit unclear, as I'm not sure what the "cylinder" is. I assume it is the laser collimation tube, but perhaps you could clarify that for me. Otherwise, this is the general method for aligning a diode laser in a Littrow configuration from scratch:
For diode lasers in a Littrow configuration, the goal is to feed laser power back into the cavity. If you knock it out of alignment, it is likely that the diffracted order from the diffraction grating is no longer going back to the laser.
To check this, use a piece of paper with a small pinhole (~ 1 mm in diameter or so) and let the output of the diode laser go through the pinhole near the output of the laser and before the grating (if you're working with this, you should know to turn the power down on the laser to just above the lasing threshold and use proper eye protection). When the laser is going through the pinhole, if the beam is not well aligned, you should see the diffracted order somewhere on the paper. You can then adjust the grating so that the diffracted order goes back through the pinhole.
Next, turn down the laser power to just below the lasing threshold. Then adjust the alignment of the grating until you see the laser power increase, or "flash", which means the laser now has enough feedback to be above the lasing threshold. After finding the optimal alignment (most stable "flash", note it need not be flashing, its just brighter than it was), turn down the current to just below threshold again and repeat until you can flash the laser at the lowest output current. At this point your laser should be well aligned again so that you can tune it using the piezo on the grating.