tried using Migration Assistant from my old iMac to new iMac. It stalled about 3/4 of the way thru. Now I don't have any photos on my new iMac. Also have ?? on the Dock where apps use to be. At this point can I run Migration Assistant again?

Yes, you can start over again but it isn't easy. The first thing you must do is reset the new iMac to its factory state as follows:

Internet/Network Recovery of El Capitan or Later on a Clean Disk

You will need a fast, working Internet connection, preferably, Ethernet. If you must use Wi-Fi, you will be asked to select your network and enter the network password. This procedure will install the original version of macOS that came pre-installed from the factory.

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) or Mac OS Extended, (Journaled.)
8. Provide a volume name, usually "Macintosh HD."
9. Click on the Apply button, then click on the Done button when it activates.
10. Quit Disk Utility and return to the Utility Menu.
11. Select Install OS X and click on the Continue button.

Once the computer finishes the installation, it will present the Setup Assistant. At this point you need to plan the next direction. You must start with getting your old iMac into a clean, working state. If you have had problems, then they need to be fixed before you start migrating. You will need to decide whether to migrate or restore from your Time Machine® backup. You do not need to connect the iMacs together in order to migrate. If you plan to use the Time Machine® backup, then be sure to connect the Time Machine® backup drive to the new iMac before you start.

Ideally, your old iMac is running Catalina, but if it isn't then upgrade macOS to the last version of macOS supported on the old iMac. This will help assure a problem-less migration. Because you are migrating your old user account, do not use the same username and password for the Admin account you will create in Setup Assistant on the new iMac. Be ready to do the migration while still in Setup Assistant rather than completing the Setup and using Migration Assistant afterwards.

The most reliable method and fastest method of using Migration Assistant is simple and I will provide the steps below, however as mentioned you need to start over and do an erase and install on your computer to start with a clean slate and that is what I'd recommend that you do.

The most reliable and quickest method to use MA is to do following:

• Connect the old Mac's Time Machine External Hard Disk to the new computer using a USB cable.
• Launch Migration Assistant.
• Follow the on-screen prompts.

Yup, its that simple.

I prefer using the Setup Assistant in a situation like this one. I'm always a bit leery about using MA. When setting up a new Mac, using Setup Assistant is even simpler:

Connect Time Machine® backup drive per your instructions

Push the Power button, follow on-screen instructions.

I agree 100% I always set up my new Macs using Setup Assistant, I think it is a smarter option than MA! I only recommend MA when users have already used SA and do not have the option to use it again.