If nobody can image computers and you just get kicked back to the BIOS read on:
The certificate for PXE Boot is renewed automatically every 2 years or so but need to be imported into SCCM.
Remote into LCSOSCCM
Open certlm.msc as an admin
Navigate to Personal > Certificates
Find the one with the Certificate Template of "SCCM Distribution Cert"
Right-Click it and hover over "All Tasks"
Click Export
Fill the prompts as follows:
The password is in Keeper under "PXE Boot Certificate Password"
Save under C:\Tools\Certificates | Name can be whatever you want
Open SCCM Console on your computer
Navigate to Administration > Site Configuration > Servers and Site System Roles
Single-Click \\LCSOSCCM01.leo.lcso.int
In the box that opens on the bottom Right-Click "Distribution Point" and select Properties
Go to the Communication Tab
Under Import Certificate, Browse to the one you just exported.
Enter the Password from earlier and click Apply.
Close all windows and try PXE Booting again.