Sounds like malware, something is getting in the way of the startup process, timing out, and then after the 10-15 minutes explorer is allowed to start. Go into task manager as soon as it boots, try killing explorer.exe process if you see it, then, do file > run > explorer.exe and see if it starts up.
Would try running malwarebytes
run a proper scan with whatever AV product is installed - if not, use Sophos rescue scanner or something.
If you can get into control panel, see if anything has been installed recently, try removing it.
Could also try system restore.