My Apple pen will not connect

The tags appended to your post suggest that you have an iPad with iOS10 installed. If this is what you have, your iPad fundamentally does not support Apple Pencil.

So, starting at the beginning - compatibility. Verify that your Apple Pencil is compatible with your iPad:

Apple Pencil compatibility - Apple Support

Assuming that you have an Apple Pencil that is compatible with your iPad, this support page may be helpful with pairing and use of the Pencil with your iPad:

Connect Apple Pencil with your iPad - Apple Support

If you continue to experience difficulties, here is one of the better troubleshooting guides:

https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/

Be aware that if your Pencil has been set-aside for an extended period, unused and uncharged, the Pencil battery may now be irretrievably damaged through deep discharge. In common with all Li-ion batteries, they must always have some level of charge.

The tiny internal Li-ion battery is susceptible to permanent/irreversible damage through being left discharged for long periods. Even some “new” pencils can exhibit signs of failure out-of-the-box if they are “old stock”. 

It is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless. Setting aside an unused Pencil, for extended periods, is a recipe for premature death of the Pencil battery.

If the Pencil Battery has failed, the only remedy is to replace the Pencil. If the battery has failed and your pencil is within its one-year warranty, you should look to having it replaced by your retailer or at an Apple Store.

Gentle warming of the Pencil (such as placing on a hot water radiator) prior to attempting charging the Pencil can resurrect an ailing Pencil battery.

When trying to pair the first generation Apple Pencil with the iPad10, are you using the special Apple USB-C to Lightning adapter to connect the USB-C/USB-C cable supplied with the iPad? If not, this may the source of your difficulty.

• USB-C to Apple Pencil Adapter

https://store.apple.com/uk/xc/product/MQLU3ZM/A

While this special adapter is included in the box of new first generation Apple Pencils, some older stock Pencils may not have this adapter in the box - but will instead have the older Lightning/Lightning adapter.

The first generation Pencil is paired using the wired connection between the iPad and Pencil.