From the article SharePoint 2013: How to change all service account passwords, this is what should have happened. I'd suggest to restore the old password first, and then follow these steps exactly:
SPAPP - Change the Application Service Account Password
- Standard domain account.
- This account is generally used as the content web application pool identity.
- May also be used as the identity for the Work Management Service Application (WMSA).
- Managed (if used for content web applications and WMSA)
- Change the password in AD Users and Computers as you would normally.
- Restart all farm servers on which SharePoint is installed.
- Next, remote into any SharePoint server using the SharePoint Setup User Administrator account (eg, spAdmin).
- Test content web applications by trying to connect to them. You should see HTML 503 Service Unavailable messages in the browser.
- Remote into a farm application server using the SharePoint Setup User Admin Account (eg, spAdmin).
- Launch Central Administration.
- Navigate to: CA > Security > General Security > Configure Managed Accounts.
- Look for the App service account, and then click the Edit icon next to the title.
- In the Credential Management section, check Change password now.
- Next, also in this section, select Use existing password and then enter the password.
- Scroll down, and then click OK. The page will be busy for less than a minute. Once it returns, move on to the next step.
- Remote into a WFE.
- Launch an elevated command shell.
- Execute IISRESET, and then wait for it to complete.
- Repeat steps 12-14 for each WFE in the farm.
- Try connecting to each of the content web applications of the farm. You may get the usual "Sorry...". This is expected and is due to time out behavior: the IISRESET initiates a complete recompile of each of the web applications on the WFEs. Keep trying.
- Once you verify the content web applications are back up, try hitting your My Site and your OneDrive. Here too you may experience errors presented in the browser. This is expected. The My Site web application was also recompiled (if they are co-hosted with your content web applications).
The SharePoint App service account (spApp) is typically used with content web applications and the Work Management Service Application (this is created and configured automatically for you during normal install). Managing this web application AppPool identity via SharePoint Managed Accounts makes changing this particular password a snap. An IISRESET is absolutely necessary after restarting SharePoint servers and then changing the password in Managed Accounts. if you don't do this, you'll experience 503 Service Unavailable errors.