Does wudu break if my hijab comes off?

Uncovering isn't a nullifier

Wudu is broken by things like using the toilet, passing wind, deep sleep and losing consciousness. Taking off clothing, or having it slip off, isn't on the list in any of the four schools. That's true for the hijab, a scarf blowing off in the wind, or even changing clothes completely. See Does being naked break wudu? and What breaks wudu?

If your hijab slipped in front of non-mahram men, that's a matter of modesty, not of wudu. Something that happens by accident isn't a sin. Just fix it calmly and move on.

Covering for prayer is separate

A woman must cover her hair during prayer, so check your hijab before you start. If it slips during the prayer and you fix it quickly, the prayer is still valid according to the scholars. If a large part of your hair stays uncovered for a long time and you don't fix it, many scholars say the prayer should be repeated. Your wudu, though, stays intact either way.

The wiping-over-the-hijab exception

In wudu, you wipe your head:

Most scholars say you should wipe at least part of your actual hair. The Hanbali school allows wiping over a hijab wrapped under the chin in some cases, much like wiping over socks. Some of those scholars say that if you remove the hijab you wiped over, the wiping no longer counts, so you would need to wipe your hair again (or renew wudu). If you wiped your hair directly, removing the hijab changes nothing. For the details, see Can I wipe over my hijab in wudu?

Practical tips at work or university

• Make wudu with your hair exposed (in a private stall or at home), wipe your hair, then put your hijab back on.
• If you'd rather not untie it, slip wet fingers under the front of the hijab and wipe the front part of your hair; most scholars accept wiping part of the head.
• Keep a mirror and spare pins in your bag for quick fixes before prayer.