What happens to a chart when the birth time is unknown

An unknown birth time is common and it is not the end of the reading. It does mean an honest astrologer will tell you what they have stopped being able to say.

What still holds

Every planet's sign, except occasionally the Moon if it changed sign that day. Aspects between planets, since those depend on positions rather than time. The general character of the chart, its element and modality balance, and any tight configurations.

What does not

The ascendant, and therefore every house placement. Anything that depends on houses, which includes most of what people actually want: career timing, relationship houses, which department a transit lands in. In Vedic astrology, the dasha timeline, because it is calculated from the Moon's exact degree.

The noon convention

Most software, including ours, casts an unknown time at noon. It is a midpoint, not a guess at your real chart, and it keeps the Moon roughly central for the day. Crescent flags a chart cast this way rather than presenting it as though the houses were real.

Rectification, and its limits

Astrologers can work backwards from known life events to estimate a birth time. Done carefully it is legitimate; done loosely it becomes fitting the chart to the story, which will always succeed and always mean nothing. Treat a rectified time as a hypothesis, not a fact.