When the toolchain id changed (for instance from baeyens.it to Sloeber.io) your project/sketch will no longer compile

To fix this you go to project properties
Select "C/C++ build"
select "toolchain editor"
You should see errors (if not this is no fix for you)
deselect "Display compatible toolchains only"
select a toolchain different from "Arduino Toolchain"
select "Arduino Toolchain" as toolchain
select "C/C++ General"
select "Preprocessor, include paths,Macro etc"
select "Providers"
Make sure "Arduino Copiler Settings" is active
If you can preSs the reset button pres it
you should se following command
${COMMAND} -E -P -v -dD -D__IN_ECLIPSE__ "${INPUTS}"
note the -D__IN_ECLISPE__
select "OK"
Verify the project.

Note: You need to do this for all your projects and for all the configurations in these projects.