Issues can happen with the mPOP registers for a myriad of reasons.
Thankfully, a quick power cycle generally fixes many connectivity issues.
How to power cycle your mPOP
Turn off register & unplug from wall
Reset iPad
Upon relaunch of C-Trax Serv, complete Check Hardware steps below
Check Hardware Mode
In your Settings, select POS hardware. Ensure you have mPOP selected.
Check to ensure your mPOP is paired
On the POS hardware screen:
Tap check POS hardware and check to ensure StarIO status says Connected
Tap select bluetooth printer, ensure your printer is paired and selected
Enter your iPad Bluetooth settings to ensure the device is connected.