Does not boot ubuntu?

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http://askubuntu.com – I was having Ubuntu when i install ubuntu i completly formated harddisk and then i made 3 partitions one for root one for home and one for data . and i left 25 Gb Memry unused. All partitions were ext 4. After some months now i have installed the Windows 8 in the free space(24GB) . now it direclty boots Windows 8 and does not ask for the choice between Windows 8 and ubuntu . I can only acces the partition in which i have installed the Windows. I want it to ask for choice between Windows 8 or ubuntu or some other way to run ubuntu or to access the data in other partitions . Because im having ve (HowTos)