Clip mask files turned into jpg files

That's very strange, and looking at your screenshot, it does appear that at least the selected calendar page has become a single embedded raster image rather than the original editable artwork.

The control bar shows a 300 PPI CMYK image, and the Links panel lists multiple image objects. That doesn't necessarily mean they're JPEGs, though.

Monika may be right about Illustrator losing the native editing information, but I'd be curious about something else. Even if Illustrator falls back to the PDF portion of the file, that doesn't necessarily mean everything should be rasterised.

A few questions:

• Were you working exclusively in the AI file, or did you reopen the PDF at any stage?

• When you saved the PDF, was Preserve Illustrator Editing Capabilities enabled?

• Are all 32 pages now individual images, including the text and calendar numbers?

• Did you use any flattening or rasterising operations before saving?

• What version of Illustrator are you using, and where were the files saved?

I'd also check whether you have an earlier version available through your backup or cloud storage's version history.

Unfortunately, if the artwork really has been rasterised and the original editable objects are no longer present, releasing a clipping mask won't recover them.

-----
Another possibility is that the artwork was rasterised during PDF creation, particularly if transparency flattening was involved.

When saving to certain PDF formats, such as PDF/X-1a, Illustrator has to flatten transparency because those formats don't support live transparency. Depending on the complexity of the artwork, overlapping objects, transparency effects and clipping masks, parts of the design can be converted into raster images rather than remaining editable vectors.

However, flattening doesn't normally turn an entire page into one image, and clipping masks alone shouldn't cause this. Your screenshot shows what appears to be a complete calendar page embedded as a 300 PPI image, so I'd be interested to know which PDF preset you used and whether you reopened the original AI file or the exported PDF.

What happened in the Layers panel? Are there too many clipping masks?

If so, your file could have been corrupted while saving and no longer has an AI part, only contains the PDF. 

This may have been caused by the option “Save in background” in the preferences. You’d better turn it off. But that will not recover the files.