If it happens overnight my guess is that although they look like caterpillars they are actually the larvae of a sawfly, the imported currantworm, aka currant sawfly, Nematus ribesii. Do they look like the larvae in this picture?
http://bspm.agsci.colostate.ed...
I had alpine currant almost completely defoliated by this insect last year and it did seem to happen almost overnight, I wouldn't have believed it if I hadn't seen it. I spent ages going over the plants almost leaf by leaf, either hand-squashing them or spraying with insecticidal soap because we cannot use anything stronger in Ontario. It did work although there was a second generation so watch out for that if this is your pest.
In the USA there can be more treatment options but it depends where you are. If this looks like what you have either post back with your country/state/province and we'll try and find out what you can use, or contact your local extension office for legal options (this is assuming hand-picking and insecticidal soap aren't for you). As well as insecticidal soap I've seen azadirachtin (neem extract), pyrethrins and spinosad suggested.
But do remember if this is your pest then things like the Bt version for caterpillars will not work because they aren't caterpillars even though that's what they look like. If you're unsure, maybe you could try and post a picture of them for us.
