Short answer: Copy-paste the HTML code rather than the rendered signature
A client of mine created a brand new signature combining images, text and links, in HTML format.
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Short answer: Find out what specific part of the boot sequence fails and, if needed, 'google' the problem
The first thing to try would be rebooting in "Safe Mode" which cleans some cache and only loads the minimum from the system to operate.
If that doesn't work, then you'll need to reboot the computer in "Verbose Mode", see where the login procedure is stuck and Google the problem
Provided that you already downloaded Pages from the Mac App Store in the past, here's how to do it:
Open the Mac App Store
Click on your account (at the bottom)
This will give you access to the "Purchased" pane.
Locate Pages and initiate the download from there.
You'll see a message telling you that the latest version is incompatible so it will download and install a previous yet compatible version.
Desktop Macs should get backed up via Time Machine (TM) to an external drive permanently connected to it.
For MacBooks, using an external hard drive is feasible (and recommended) but not super practical.
Another solution for MacBooks (especially those without a built-in SD card slot) is to invest in a Network Attached Storage (NAS) solution, like those from Synology.
Entrepreneurs or small business owners who have a Mac laptop that is about 4 years old could buy a new one (through their company) then reset their old one and give it to their child.
Another option, depending on your country of residence, could be to check the refurbished section of your online Apple Store (it if exists):