Start with the basics before blaming the starter. Is the starter cable 00 gauge? Are the connections perfectly clean without corrosion or even light oxidation?
Check ALL your ground connections to and from the battery to the engine, cab, and chassis.
How old is the battery? They can and do fail sometimes after working just fine one minute and going dead the next.
When you say it worked fine last week, what does that mean? Did it spin the engine over quickly like modern car? Did it drag along slowly and just happen to catch?
