If a reboot doesn't help, then try booting into Safe Mode which prevents third part software from launching automatically during boot & login. See if Safe Mode allows you to get the installer.
You may want to run Disk Utility First Aid on the hidden Container in case the file system is damaged. Withing Disk Utility you may need to click "View" and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok", click "Show Details" and scroll through the report to see if any unfixed errors are listed. If there are any unfixed errors, then you will need to erase the drive and reinstall macOS and restore from a backup. Erasing the drive destroys all data on the drive.
Otherwise, it may indicate you have a hardware issue of some sort (memory, hard drive/SSD).