First rule out a medical condition that is causing your child to have bladder control accidents. It is somewhat common for children to have infections as they don't always have the best hygiene.
If your daughter does not have an infection or another medical reason it there might be other explanations. One could be that she is engrossed in her activities and simply forgets. Perhaps she had a change in routine? Maybe there is some physical change in the home, such as a new toilet seat that she perceives as "scary". Is there something new about your home situation, such as a new baby.
Keep in mind that many children "backslide" with their training and this is not usually of great concern. But, once again, you may wish to first rule our medical reasons.
I think it's pretty normal for kids to experience little set-backs like this while they're potty training. I think sometimes it happens when something upsetting or stressful happens in a little one's life (like a move to a new house, starting school, big changes in the family like a new baby, divorce, or re-marriage, etc). But I think it can also just happen as part of their normal growth and development pattern.
I think the thing to do is to stay really positive. Let her know that it's ok that she had some accidents and that she's still a "big girl" and can go right back to going in the potty. Don't punish her or make her feel bad about it. Just use positive encouragement to get her back on track. If she continues to have accidents, you may even want to go back to any positive reward system you used the first time around (a sticker chart for example).
it is normal for children to have set backs when they are potty training. It could be in relation to other events eg starting a new school or it could be for attention seeking. If she does have an accident try not to make a fuss but do reassure her that she won't have to go back to the potty. Praise all the times she does make it to the toilet and make more of a fuss of this than the accidents. Children generally respond to the praise when they get it right rather than the negative attention when they get it wrong. Be practical too in the early stages and encourage regular trips to the toilet.