What breeds/sizes/ages are the others? How long has your roo been gone? Has this flock been established for awhile, or do birds come and go? How long have the girls been tormenting your bantam hen?
Sometimes the roo is the lynchpin holding a flock's pecking order together, and his removal will start the hens fighting among themselves for a place in the new pecking order. A good roo keeps his girls in line so everyone feels secure in their place and there is no fighting (there are bad roos too, who are only concerned with their own food, water, and satisfaction, but most roos will keep the peace b/c flock safety is easy to monitor if no one's running and squawking and feathers aren't flying). You may end up having to get another roo if you can, before you can re-introduce your poor girl.
Some breeds are more aggressive than others (Cochins are usually very docile) and bantams tend to have lower slots in the pecking order because of their size (game banties might be lower, but they sure don't act like it). A flock that's well-established, with fewer chickens coming and going is actually likely to adjust to flock changes more smoothly than a flock whose population is always changing. The flock in flux may act more dramatically and peck newcomers more aggressively. Sounds like your flock just went all to heck w/o the roo, and now everyone is unsure of their position in the order & aggressively sorting it out - which sometimes means ganging up.
So, after all this blahblahblah, here are some things that might help:
Your poor little hen's immediate safety is job#1. You'll probably need to cage her up and bring her into your house if the barn's not got a secure place for her cage. Keep her safe and warm (using a heat lamp or putting her in a warm place) for a few days, to let her system recover from the stress. Some electrolytes in her water may be a good idea (they make special electrolyte mixes for chickens, like Sav-a-Chick, etc.). Give her a treat as well. Quiet, warm, secure, electrolyte, treat: these are just the things a harried hen needs.
If you can, replace the rooster who's gone with another rooster and give him a little time to sort out the hens (once he's walking around like he's at home and when he can tread a hen, then things should be stabilized there). If you can't replace him or want a roosterless flock, you'll need to give the more aggressive ladies extra time to sort out who is where in the order before you can safely bring your poor girl back in. When the big girls have gotten themselves settled, it will be only the lower-ranked chickens that peck on your hen to make sure she knows she's at the bottom, and the top hens won't really participate.
Re-introduce your bantam hen slowly, like putting her cage out in the yard (if your chickens free-range) or bring her cage closer and closer to their pen (if they don't), so they get used to seeing her. You can put her cage inside the pen if the pen is big enough. When you can be around to watch them, let her out of the cage so she's back with the others. They may chase and peck a little, but they should stop (except for an occasional "warning peck" when they feel she has overstepped herself) when she takes a submissive posture (front crouching, head down). A distraction for them might reduce their aggression, such as a treat that keeps them busy - a flock block, a forage cake, a 1/2 of a cabbage for them to peck. I would also recommend that you have some blood stop powder on hand just in case. Blood stop powder makes a rubbery black seal over open wounds so they heal and don't attract unwanted pecking by the other chickens. Blue-kote works in the same manner.
I hope that this helps! I have a multi-breed flock with a single Cochin bantam myself, but my old Delaware roo and hen run a pretty tight ship, pecking order-wise. My Cochin bantam (a Mille-Coch named Carmelita) is kind of mean, so I guess she holds her own. Keeping my fingers crossed for you.