Engine overheated. Won’t start, but turns over. Does not fire at all. Could it be a slipped timing belt? Has 99,000+ miles.
Why would you assume that when it overheated? Has the belt not been changed before?
How badly did it overheat? Why did it overheat - if you know? Was the check engine light on before it overheated?
So you say it turns over - you mean the engine turns when the starter engages? But won’t start? Have you checked for spark? If you spray a little starter fluid into the engine when your assistant cranks it over and it runs only while you are spraying, it is a fuel issue.
When an engine overheats to the point where it will not start there is usually damage to the engine. In most cases the cylinder head(s) is warped and there is no compression.