When a campaign ends or is made inactive, you can transfer its funds to another campaign of your choosing.
To Transfer to an Existing Campaign
Go to Manage-> Campaigns, click the Options button next to each of your old campaigns and select Transfer Funds. Follow the on screen instructions to initiate the transfer.
To Transfer to a New Campaign
Go to Manage-> Campaigns. Create a new campaign like you normally would. Enter your budget, credit card information, etc.
Before you save, click the "Set to Inactive" checkbox. Then save. The campaign will be created, but you will not be charge and the campaign will not be funded.
Then, back in Manage-> Campaigns, click the Options button next to each of your old campaigns and select Transfer Funds. Follow the on screen instructions to initiate the transfer.
Once the transfer is complete (usually about one business day) you can activate your new campaign and it will run with the transferred funds. Just make sure the funds cover your budget amount or you will be charged.
For example, if you set a $100 budget and transfer $75 into the campaign, you will be charged $25 when you launch.