My bobbin lock...the black piece..on my singer 401a came out...how to put it back? Thank you

Page 72 of the full manual may help, if you didn't have this manual when you bought your machine.
Hope this helps.
https://www.singer.com/search/support?title=5705

They have an unusual loading bobbin carrier! Watch the video from a singer technician.

https://www.youtube.com/watch?v=Rkl0xuD6C3E

also make sure your bobbin tension is correct, as it will bird nest if too loose.

https://www.google.com/search?q=bobbin+tension+explained

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Please download the user's manual for your machine free at the Singer site, under support. On page 50, it shows you how to remove and replace the bobbin case. I have a feeling that it's not quite in place, and that's easy to do. Ask me how I know.

the gears or some part in the sewing machine could be damaged, I suggest having the machine checked to see what is wrong inside the machine hindering it from sewing.

Go to the singer website
Go to the home page
Go to customer service--click on it--it will show a small menu--click on instruction manual
In the box put the model number (example 401a) & click search

There's no technical part called a "bobbin wheel" so I am assuming you mean the shuttle where the bobbin case goes.1.Unplug your machine2.If you look closely at the area where the hook came from, you'll notice there's a curved piece called the "driver" which moves in an arc when you turn the hand wheel. The hook is also curved. If you turn the handwheel until the driver looks like a smile, you can hold the hook by the center pin and turn it to look like a frown. If you put the frown on the smile, it will fit and all you have to do is put on the cover piece and turn the 2 pivoting holding arms over the cover until they lock into position and you are done. Put a little oil on the hook before sewing for best results. I hope this helps!Jimmy

Something is not threaded right on the top, or the thread is tangled. Take the thread completely out and rethread. Also, make sure that no thread is caught in the bobbin area, if necessary take bobbin casing out and clean with a brush. Always turn the handwheel towards you, never away from you because it can mess up the timing, Also google Singer 401a there is a great blog on this machine

Here's a manual for it on this site. www.ismacs.net/singer_sewing_machine_company/manuals/singer-404-singer-slant-o-matic-sewing-mac

blog.sew-classic.com/2008/10/25/singer-401-401a-403--403a--404--sewing-machine

These two sites may be of some help.

Cheryl
401A Singer

I'm having a similar problem today. I took out the bobbin case and when I went to put it back in, the whole system locks up. I've tired cleaning, changing needle taking the metal plate and piece that holds bobbin case out and put it all back in after cleaning. Nothing. My symptom is I can't get a complete stitch, the needle goes down, the whole deal acts like it will work and as soon as the arm of the bobbin case gets to 8 o' clock on the gears, It jams and won't hardly move. I cannot pull the bobbin case out easily after this. Perhaps there is someone out there that can help us!

to wind a bobbin you get thread and put it where it usually would be put it behind those 2 metal piece (see pic in manual) and there put it around the thread guide and then u put the tip of the thread through the hole of the top of the bobbin from the inside to the outside  and u are really to start, u should already have the bobbin and the spinning metal piece, then u move the metal piece to the right into the notch and press the presser foot and hold it until it stops you and you're done :-)