You mean that Dropbox won't connect? Do you have any internet at all?
You can open your System Preferences, then the Network pane.
If you are using ethernet (wired), then you should see your internet settings in that pane.
Click on the "Assist me..." button, then click "Diagnostics..."
Are all the indicators in that Diagnostic screen green? If not, click continue and follow the steps.
If you search for "Dropbox no internet connection OS X" - you will find quite a few links for possible fixes. Looks like a common one is making sure that your proxy server settings are correct. (if you don't have a proxy server, then any proxy server settings should be off)