When you flash your micro:bit over a USB lead, it will remove all record of any previous Bluetooth pairing on the micro:bit itself. The phone or tablet you used will still remember the pairing, but this will now be invalid.
You'll need to add the Bluetooth package to test. If you are NOT using the Bluetooth package, you MUST enter "Pairing Mode" before the "Connect" button will work. Here's an example program that enables many Bluetooth services.
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