OS 3.0 doesn’t have a microSD card, instead it logs onto the flash memory space in the microcontroller. The space is not a lot (1MB by default but there is actually 3MB available space for storing data). My suspicion about logging stopped working is that perhaps it exceeded the available space (though even with 1MB it can store quite a lot of data before running out of space).
If you have a previous version of OS, like OS 2.0, 2.1, 2.2, since they don’t come with a pre-installed microSD card, it may be that the SD card slot has a connection issue. For OS 2.3, it comes with pre-installed microSD card and that has been tested so logging should be working on OS 2.3.