Do you mean you erased the Macintosh HD - Data volume or that you deleted the volume in Disk Utility?
If the former, then the volume is still present. You can restore its content from your backup. However, if it's the latter, then you need to reinstall macOS. If you have no backup, then your data are gone, and you need to reinstall macOS.
Install macOS from Scratch
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
- Select Disk Utility from the Utility Menu and click on the Continue button.
- When Disk Utility loads select the target drive (out-dented entry w/type and size info) from the Device list.
- Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
- Set the partition scheme to GUID.
- Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
- Click on the Apply button, then wait for the Done button to activate and click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.