If you haven't tried this already, hook it up to a display, not just SSH or VPN, because it may not get that far in the boot process. Then on another (if possible, new) SD, install a fresh OS image, raspbian or noobs probably. Try to boot from that while the monitor or tv is hooked up to the RPi (make sure if its a TV it is on the right input). If it is black, there is a very bad problem. If it works perfectly, than the other SD is bad or the OS on it is corrupted, I've had both happen before. If the OS is corrupted, it's possible to restore some of the data. If nothing recognises it, like if another linux machine doesn't . If something shows up, but it does not run normally, post any error messages you can find. If it is black or there is a booting error, try using a multimeter to check the charger amperage. It needs to be minimum of 600 mA according to RPi B+ docs, but I have found 1500 mA to be much more reliable, sometimes even necessary if using GPIO. Check it with the multimeter, not just looking at the label, it may be a bad power supply. Many phone chargers do not supply adequate power. Most all Apple chargers do, but other USB adapters often do not. If you want to buy a cheap power supply, I recommend the official one. I have it and it works great, even powering a small cooling fan. Next, check the voltage of the power supply with the multimeter. It should be near 5 volts.