I recently dropped my Canon EOS 600D attached to the 18-55mm IS II lens inside the Canon camera bag from a height of 3ft, and the zoom ring after that is really tight, and I hear a clicking sound when I rotate it, but the auto focus seems to work fine. What can I do now?
Replace it with another lens. Warranty coverage includes defects in materials and workmanship, but does not usually extend to user damage.
You can pick up a used EF-S 18-55mm IS II lens at Adorama for about $80-125, or a new one for about $200 on Amazon. It would probably cost more to repair the one you've got. Other options would be lenses such as:
Tamron 17-50mm f/2.8 Di II (non-VC or VC version)
Sigma 17-50mm f/2.8 EX DC OS HSM
Sigma 17-70mm f/2.8-4 DC Macro OS
Canon EF-S 18-135mm f/3.5-5.6 IS
Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens
Canon EF-S 17-85mm f/4-5.6 IS USM Lens
If you can afford it, this is a good chance for you to upgrade to a worthy lens, with bigger aperture.
The cheapest option is Sigma 18-50 F2.8-4.5 HSM OS. $250. Even better is fixed aperture F2.8 for 600$, but as you can see the price jump is substantial.
I have heard good about Tamron's fixed aperture version 17-50mm F2.8 for $500 (not VC, which is significantly worse), but in general I am not impressed by their build quality - I have held more than one Tamron's (owned by friends) that after 1-2 years go "crunch crunch" when you focus it. Never have I seen that with Canon and Sigma, or even 30-60y old vintage lenses.
People worry about non-Canon AF performance, but my Sigmas perform equal or better than my Canons.
Well. There's a few things you can do in cases like these:
You can keep using it until it breaks well and truly (there's a small chance it will cause damage to your camera though if something breaks off and scratches the sensor say),
You can get a quote for repairs from canon or a 3rd party (considering the cost of a new(or secondhand) lens i wouldn't advise it)
You could try and repair it yourself, if the lens is a write off it might be an interesting project
You can sell the lens as 'for parts' online and get a few bucks for it, maybe someone else has repair skills of wants to use parts in a project.
You can buy a replacement or you can upgrade
Hope that helps!