Here is information from the Dell Knowledge base you may find helpful:

<ADMIN NOTE: Broken link has been removed from this post by Dell>

Touchpad Usage and Troubleshooting Guide

and a short video:

How to Fix Touchpad Problems on a Laptop

You can also run Dell SupportAssist to detect any hardware issues.  Make a note of any error messages encountered and post back.  Thanks.

My point in my reply was, thanks for the above tips, but none of them solve the issue. Something is wrong with the Laptop itself, or the updates that cause the issue, which explains, 1) why the first time my touchpad stopped working and then, began to work after an update, but 2) not why the touchpad stopped again, unless there is a missing update, and 3) why an external mouse does work but not the laptop's touchpad.......

I  have had 5 touchpad failures within a year. Dell replaced my laptop with a refurbished model after waiting over 4 weeks for a second new touchpad replacement they kept backordering. The touchpad on the refurbished model failed and was replaced only a few weeks later. The 5th touchpad has just failed after three weeks and again they have backordered the replacement pad. The last technician that replaced the part told me he has many service calls for this same issue. Soon the warranty is going to run out and Dell is just running out the clock until that time. Then I will be stuck paying for more touchpad replacements. This is the worst laptop and service I have ever had (and I've had many computers).

it is mechanical. the touchpad on the inspiron 7420, purchased jan 2023, has intermittent frozen mouse.
i would squeeze right of pad to get it going again --which releases contact of the click mechanism.
the click mechanism is a tiny disc affixed to the logic board. it is center bottom of the pad. the design is faulty or the disc is faulty.
the touchpad is the only bad of the product, everything else about the laptop is a good bargain.

@vanung​ Exactly where do you click?

This is my last Dell laptop. They know they have an issue. They don't fix it properly. 

I bought a cheapo laptop that at least works. I had to do that because I made the mistake of buying a Dell.

I purchased the Inspiron 15 3000, and after just one month, the touchpad began causing significant issues. It became apparent that this touchpad issue is widespread among Dell laptop users. Regrettably, this experience has led me to conclude that this will be my last Dell laptop. I made a substantial mistake by choosing a Dell product

I have been experiencing this very same touch pad problem, without a fix. I strongly feel that these laptops are not in good quality.

Same issue here, my fiancee's Vostro 15 3515 had intermittent touchpad issues from the outset.  A touchpad firmware update seemed to settle it, but one month out of warranty it's just locked up again.
Symptoms:-

• Can click, can navigate, cannot click-and-drag (e.g. cannot drag the corner of a window).
• Repeated attempts to click and drag will crash the touchpad, but not its buttons. You can click, but you can't move.
• Prior to crashing, movement may get jerky as though your finger isn't reliably contacting the touchpad.
• A mouse, plugged into the laptop, is not affected.
• Putting the laptop to sleep by closing the lid, then waking it up (effectively power-cycling the touchpad) reactivates it.

As an aside the laptop also seems to run hot, as the fan is noticeably noisy, ALL the time. It only shuts up when it's sleeping.
It's just as well she wants to keep the laptop, as with a defective touchpad its resale value is zero.

Add me to the list of people with a stuckj trackpad. Concerning that Dell is not addressing this seemingly widespread issue