All of a Sudden Unable to Open PhotoShop

Did you purchase CS6 when it was still available?

CS5 and CS6 are on life support. But they can still be activated providing you have the following:
1. your original media (disks or downloads) you received at purchase and a way to read them,
2. a legitimate serial number for that version, language and platform (Win/Mac) and if it was an upgrade, a qualifying serial number from an older product,
3. an older operating system that can support legacy 32-bit apps.

Before you install CS6, run the Adobe Cleaner tools to remove apps from your system.

https://helpx.adobe.com/creative-cloud/kb/cc-cleaner-tool-installation-problems.html

NOTE 1 : Creative Suite installers won't work on macOS Sierra, High Sierra or Mojave without terminal workarounds. See the help article below.
https://helpx.adobe.com/creative-cloud/kb/install-creative-suite-mac-os-sierra.html

NOTE 2: Legacy Creative Suite will never run on Catalina or Big Sur. Only the latest Creative Cloud products are compatible.

NOTE 3: Legacy Creative Suite is discontinued & unsupported. Use at your own risk.

For a subscription

Revert to trial https://helpx.adobe.com/manage-account-membership/cc-reverts-to-trial.html may help
-and troubleshooting FAQ https://community.adobe.com/t5/Get-Started/Troubleshooting-FAQ-What-should-I-do-if-I-have-a-membership-but/td-p/7601068
-and https://helpx.adobe.com/creative-cloud/kb/cloud-website-shows-trial-purchase.html

To download an older subscription version for your operating system

Direct Download Links https://prodesigntools.com/tag/ddl MAY help
-you MUST do what it says in Note: Very Important Instructions
-https://prodesigntools.com/adobe-cc-2015-direct-download-links.html#download-instructions
-the prodesign links go directly to Adobe's file server, so this process will
--only work until Adobe completely removes the old version(s) from the server
.
IF/When you do have a download, be sure to copy it to AT LEAST one external device
-since Adobe does sometimes remove old versions from the server
-a usb flash drive or external hard drive, or write to a DVD
-plus a text file containing your serial number if you have CS6/CS5

For CS6 read 

-Cloud takes over https://community.adobe.com/t5/Get-Started/Get-off-the-cloud/td-p/8075485 for some ideas

-As soon as you install any part of the Cloud, even a trial, it replaces your program management system
-if you plan to keep using a serial number program you should NOT install a Cloud program, not even a trial

Lost information https://community.adobe.com/t5/Premiere-Pro/Find-serial-number-without-Adpbe-ID/td-p/7769446
- page has links to programs to find an installed serial number in Mac or Windows
- Also https://helpx.adobe.com/download-install/kb/find-serial-number.html
- when you find your serial number(s) write them down and put in a desk drawer

I'm being told that I don't have an active plan anymore, even though I do. I upgraded last month, but it's all under the same account.

By @robschamberger

What did you upgrade?

Do you have a subscription or a perpetual licence?

What is your OS and version?