Well i've decided to update the sdk using sdk manager. I've updating at the last android M preview API. After that, i re opened Eclipse and all my projects had errors!! The appcompat library returns in the console this:
android-support-v7-appcompat] /android-support-v7-appcompat/gen already exists but is not a source folder. Convert to a source folder or rename it.
I really don't know what it means! But i can't work now!
On
i will post another answer and not editing the first one because it's another different approach: you can Undo update your sdk for this you can: Download the older ADT plugin manually (the one who was working fine, don't forget to check if you haven't it already in your downloads folder). Then copy the tools folder from there and replace the tools folder in your SDK installation directory. Your SDK will be back to the older working version.
the problem is in the ADT you must update it to a version above 23... it seems that they (Android guys) are making our life harder, they want us to migrate to their tool (Android studio) you can follow those steps:
first of all download the ADT Plugin zip file and do not unpack it. then, Start Eclipse, and select Help > Install New Software. Click on " Add ", in the top-right corner and in the Add Repository dialog, click on " Archive ". you must select the downloaded ADT-23.x.x.x.zip file (the_latest_one_available) and click OK. After that enter "ADT Plugin" for the name and click OK. In the Available Software dialog, select the checkbox next to Developer Tools and click Next. Then in the next window, you'll see a list of the tools to be downloaded. So you will click Next. Accept the license agreements (no one read it :-) ), then click Finish. Note that if you get a security warning saying that the authenticity or validity of the software can't be established don't care about that, just click OK. After completition, restart Eclipse and Voila, it must be done.... I hope so...