Probably. The most liely thing in this case is that one or more cells has been drained too far, so the BMS has shutdown and disconnected the cells from the input and output to prevent possible damage to them that could later lead to a fire. (since recharging cells that have dropped to far has that risk, and one can't know if it might happen until it does).
If is often possible to recharge the pack using the discharge port, but doing so means the cells are being recharged from a voltage lower than the cells were designed for (see above). Some people have been able to recharge individual cells directly with the pack opened up, again with the above risks. Or, again with the same risk, using a very low current (few dozen mA) charger at the minimal pack voltage (so it "can't" overcharge a cell), to recharge via the discahrge port.
The caveat with charging via the discharge port, especially with no access to the individual cells to check their voltages, is that the BMS cannot stop charging when a cell reaches full, so overcharging a cell is possible (and this has even worse fire risks than recharging cells that went too low--I don't know what added risk there is when *both* have happened
).
The battery might be "potted" to prevent environmental intrusion. That makes it a lot harder to repair, however.
That indicates the BMS has disconnected the cells, so what you read is a "ghost" voltage. This is because the FETs are not perfect switches, so there is always a tiny leakage current, which allows a slight voltage to appear, which is drained off by the multimeter's load as it reads the voltage.
A tiny charge can remain in whatever capacitors are in the system as well, whcih drains under any load.
Those are probably not internally connected, best guess.
It's almost certainly the cells; probably all of them need to be replaced (depends on how far drained they are--they're certainly drained belwo the BMS's safety cutoff for charging since it didn't recharge at all overnight). If you have no tools/etc it will almost certainly be cheaper to replace the pack than repair it.
Depending on the limitations of space you have with your bike, and whether you want to fit the new one in the same space as the old, you can probably use one of the cheap ex-EV modules from GreenTecAuto--some of them are less than a couple hundred bucks and include free shipping to at least most of the USA. Since your pack is 54v full, that makes it a 13s. You may be able to use a 14s or 12s as well; depends on your controller's design (and any LVC / HVC it might have).
