An Intel Mac that opens Windows may still have its macOS installation available. If this is a computer already configured with Boot Camp, first try the supported startup selection. Do not delete a partition simply because the expected desktop did not appear.

This guide concerns an existing Boot Camp installation on an Intel-based Mac. It does not apply to a Windows virtual machine or Apple silicon. Save work in Windows and finish file transfers before restarting.

Choose the existing macOS volume

In the Windows taskbar notification area, show the hidden icons and open the Boot Camp menu. Choose Restart in macOS. Apple notes that older Boot Camp software can instead report that it could not locate the OS X boot volume.

The alternative is to restart and hold Option, also labelled Alt on some keyboards, as the Mac begins restarting. Release it when the available startup volumes appear. Select the known macOS volume and press Return. Apple uses Macintosh HD as the example name; use the name belonging to your installation.

If the expected volume is absent, stop at that observation. An absent startup choice needs investigation; it does not authorise formatting a different volume or removing Windows to make space.

Confirm which system opened

After login, check that the expected macOS account and a familiar harmless document are present. Record which selection worked and any earlier message. Those details distinguish a startup-choice issue from an account or missing-data problem.

If you need to return to Windows later, the same Option startup selector lets you choose the existing Boot Camp volume. Save your work first in whichever system is running.

Keep any follow-up update or repair separate from this selection test. Reaching the correct desktop is useful evidence, but it does not establish that both operating systems are updated, backed up or suitable for every intended application.

References: Apple: restarting between macOS and Windows.