On a Windows server, changes made to a website's database often require a recycle of the site's application pool. For shared hosting customers, this action is performed within the Control Panel. Dedicated server customers can perform this action within IIS itself. 

To determine with version of Plesk you will be working with view How to Determine Which Version of Plesk Control Panel is Installed on a Server.

Use the Plesk 11.x Control Panel to Recycle a Website's Application Pool

1. Log into your Control Panel.
2. Choose  the WEBSPACE tab, and SWITCH TO WEBSPACE option for the domain in question.  
3. Under Websites & Domains tab expand Show Advanced Operations. 
4. • ​
5. Click on Dedicated IIS Application Pool for Your Websites.
6. Click on the Recycle button. (or stop, then start)

Use the Plesk 9.x Control Panel to Recycle a Website's Application Pool

1. Log into your Control Panel.
2. Click on Home on the top of the page. This will take you to the welcome screen of your subscription. (If the correct domain isn't listed, choose the subscription for it from the drop down menu in the upper right.)
3. On the welcome screen, click on Domains. Then select your domain.
4. Under Additional Tools select IIS Application Pool. 
5. Select the Recycle button to recycle the application pool.

Recycle the Application Pool Directly through IIS for Unmanaged Server Customers Only

1. Use Remote Desktop to log into your dedicated server.
2. Click on Start > Administrative Tools > Internet Information Services (IIS) Manager (or IIS 6).
3. Expand the server listed. Click on Application Pools.
4. Locate the pool that needs to be refreshed. Right click on it.
5. Click on Recycle.