I suspect the bobbin thread has escaped its tension spring. If you give the bobbin thread a quick short tug and the bobbin spool spins then likely the thread is not behind the tension spring. This usually causes hook jam but not always.
First be sure you have threaded per machine instructions... see below.
Next turn handwheel and position needle at the high point of stroke (always when loading bobbin). Remove and re-insert bobbin. Now, to be sure the bobbin thread is fully positioned behind the tension spring, before you move the bobbin thread into the notch, place your right index finger on top of the bobbin spool to prevent the bobbin spool from turning. Hold thread snugly (in slight tension) as you move it towards the back of the notch. You should feel the thread slide in place behind the spring. Let loose the spool and lightly pull thread. Note the resulting change in tension. (don't forget to set top thread tension back to a more normal setting)
Comment here if you require further assistance.
If you need... your manual may be downloaded at no charge from the Janome site.