When putting in a new roll of (35mm) film, I accidentally told my camera to rewind, rather than load the film. Now there's no tab sticking out of the roll, so I can't load it.
Is there a way to unwind the film slightly, so that I could still use that film?
There are film leader extractors (that at least used to be) available. I'd guess even now most decent camera stores should have one around to handle situations like yours. If you want to buy one, it looks like B&H still lists them. I would guess most others do too, though I haven't looked. In this case, it's probably cheaper to buy it locally if you can -- the price is so low that the shipping will probably double the cost unless you buy it along with something else.
Alternate to the reel puller: In a blacked out room or dark bag, open the offending canister and place the film into a reusable / bulk loading film canister.
I've been in exactly the same situation (I assume your film case is made of thin sheet metal). Being just an amateur, I had no darkroom so I did this at night with heavy curtains in my window, lights off except for a little flashlight covered with some red cloth, so i just could barely see what I was doing.
The point is, slightly opening the film case precisely where the film comes out, I used a non-sharp knife (the kind used for eating, not for cooking), then spin the spindle back and forth. Listen carefully to know when the film lead passes the opening (it makes a subtle clicking noise when you spin it in the winding direction), in this point start spinning it in unwinding direction.
If the film lead does not come out on its own, use the knife to guide it while unwinding.
Just be careful not to open the case too much, so when you're done, press with your fingers to close it again.
In My case, I had taken some shots (with a film "point an shoot") but I remembered how many, so I took again, the same amount of shots but inside a dark backpack in the darkened room, plus one, And then continued shooting normally.
Result: Somehow I ended shooting 38 exposures out from a 36 exposures film roll, wasting half a frame due to the extra dark shoot (Otherwise the shoots before and after this gap would have been half overlapped).
Good luck!
If you don't want to buy a film leader extractor, you can unwind the film using sticky paper as described here.
You can also use the film leader from another canister, as shown in this video. The steps are as follows:
If it doesn't work on the first time, don't hesitate to repeat all of the steps above.
